1. Field of the Invention
The present invention relates to an anode substrate, a secondary cell in which the anode substrate is used, a resin composition for use in forming the anode substrate, and a method for producing this anode substrate. More specifically, the present invention relates to an anode substrate which can provide a battery that is superior in charge and discharge cycle characteristics, a secondary cell in which the anode substrate is used, a resin composition for use in forming the anode substrate, and a method for producing the anode substrate.
2. Related Art
Conventionally, research and development of batteries having both a high output voltage and a high energy density have been extensively advanced. In particular, secondary cells having a low internal resistance, accompanied by less lowering of cell capacity due to charge and discharge, and that are superior in charge and discharge cycle characteristics have been demanded. For example, a lithium secondary cell in which microcrystalline silicon or amorphous silicon in a state of a thin film is used as an anode material (anode active substance) has been known (see, Patent Document 1). Specifically, a lithium secondary cell in which an anode is used that includes an anode material layer composed of a silicon thin film formed on a collector is disclosed. For forming the silicon thin film, thin film formation techniques such as CVD methods (may be also referred to as “chemical vapor-phase growth method”, or “chemical vapor deposition method”) and sputtering methods have been used.
In such techniques, materials such as silicon are considered to be repeatedly expanded/contracted as lithium is stored/released. Since adhesion between the collector and the anode material layer is high in the anode in which a silicon film is formed on the collector, the expansion/contraction of the collector occur frequently with the expansion/contraction of the anode material. Therefore, charging and discharging may be accompanied by irreversible deformation such as shriveling on the anode material layer and the collector. In particular, when a metal foil that is highly ductile such as copper foil is employed on a collector, the degree of deformation tends to increase. When the anode is deformed, the energy density of the battery may decrease due to an increase in electrode volume, thereby allowing a heterogeneous electrochemical reaction. In addition, while the expansion/contraction is repeated due to charging and discharging, the anode material may be pulverized and detached from the collector, or may be detached with keeping the form of a thin film in some cases. Accordingly, the charge and discharge cycle characteristics of the battery may deteriorate.
An exemplary method for inhibiting the deformation of the anode includes a method in which a material having superior mechanical strength such as high tensile strength and elastic modulus in tension is used as a collector. However, when an anode material layer composed of an anode material in the form of a thin film is formed on a collector composed of such a material, the adhesion between the collector and the anode material layer may be insufficient, whereby satisfactory charge and discharge cycle characteristics may not be achieved. Therefore, Patent Document 1 discloses a technique to inhibit occurrence of shriveling and the like while inhibiting detachment of the anode material in charging and discharging by providing a middle layer composed of a material that alloys with the anode material, and using a collector having a mechanical strength greater than that of the middle layer. Specifically, a copper layer is used as the middle layer, and nickel foil is used as the collector.
In addition to Patent Document 1 as described above, a technique is disclosed in which a thin film formed by a solid solution of copper on silicon is used as an anode material layer to control the quantity of storage of lithium, thereby inhibiting expansion of the anode material in the case of storage of lithium (see, Patent Document 2). Furthermore, a technique is disclosed in which an alloy thin film composed of a metal that alloys with lithium, and a metal that does not alloy with lithium are used to control the quantity of storage of lithium, thereby inhibiting expansion of the anode material in the case of storage of lithium (see, Patent Document 3). Specifically, as a metal that forms an intermetallic compound or a solid solution by alloying with lithium, Sn, Ge, Al, In, Mg, Si or the like is used, while Cu, Fe, Ni, Co, Mo, W, Ta, Mn or the like is used as the metal that does not alloy with lithium.
Additionally, a technique is disclosed in which deformed parts having a deformation amount of 5 μm to 20 μm in a depthwise direction are formed in an amount of 10 or more per cm2, and a collector having an opening ratio of no greater than 4% provided due to the presence of the deformed parts is used, whereby deformation of the electrode resulting from charging and discharging is inhibited (see, Patent Document 4). Moreover, a technique in which a material having no storability of lithium is provided on at least one of the surface and the interior of an anode material layer in the form of a thin film which can store/release reversibly lithium is disclosed (see, Patent Document 5).    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2002-083594    Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2002-289177    Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2002-373647    Patent Document 4: Japanese Unexamined Patent Application, First Publication No. 2003-017069    Patent Document 5: Japanese Unexamined Patent Application, First Publication No. 2005-196971